Materials containing sulfur at the sulfane level (thiosulfate, per-and poly-sulfides, polythionates, protein-associated elemental sulfur, and non-heme iron-associated sulfur) are ubiquitous in biological systems. Critical examination of the evidence relating to the metabolic roles of sulfanes leads to three plausible working hypotheses: (1) Sulfanes are produced in liver and kidney tissue primarily for biosynthetic use in all tissues as sulfur precursors of functional non-heme iron-sulfur proteins vital to respiratory and other oxidative processes; (2) Sulfanes are important in maintaining protein structure in many biological materials, including both iron-sulfur and non-iron proteins; (3) Sulfanes are of major importance in their detoxification role in relation to the suprisingly high natural incidence of the extremely poisonous cyanides in nature. These possible roles are not necessarily mutually exclusive. We propose to test them by several approaches that involve kinetic analysis, physical biochemical methods, and S35 tracer techniques as well as the procedures of sulfur chemistry. These studies will include close examination of the sulfurtransferases and sulfane reductases and their catalytic intermediates. The protein-associatated elemental sulfur found in biological systems will be studied as a possible transport form of sulfanes. Studies of the quaternary structures of sulfane-binding proteins will also be carried out.